2.1 IFN Therapy for Treatment of Viral Infection
Interferons are extracellular signaling proteins which have many diverse effects, including immune stimulation, tumor inhibition, and cell multiplication reduction. Interferon induces host cell production of many gene products, including the Mx proteins, the 2′-5′ oligoadenylate synthetase and RNase L. (reviewed in Sen et al., 1992, J. Biol. Chem. 267:5017–5020; Sen et al., 1993, Adv. Virus. Res. 42:57). In addition, PKR, a cAMP-independent serine/threonine kinase, is one of the greater than 30 INF-induced gene products (Meurs et al., 1990, Cell 62:379–590). These proteins have been shown to have a protective effect against viral infection, thus contributing to the beneficial effects of IFN therapies in treating viral infections.
There are many mechanisms by which IFN-induced gene products provide a protective effect against viral infection. For example, the IFN-induced double-stranded RNA-dependent enzyme, 2′-5′ oligoadenylate synthetase activates an RNase that cleaves cellular and viral RNAs, thereby inactivating viral replication, such as picornavirus replication (Kumar et al, 1988, J. Virol. 62:3175–3181). The IFN-induced double-stranded RNA-dependent protein kinase inhibits virus and host cell protein synthesis by phosphorylating and inactivating the α subunit of the translational initiation factor eIF-2α. IFN also has inhibitory viral effects at different stages of the viral life cycle. IFN inhibits uncoating of hepatitis B viral particles, thereby inhibiting viral replication. IFN inhibits the penetration of vesicular stomatitis virus into cells (Whitaker-Dowling et al., 1983, Proc. Natl. Acad. Sci. 80:1083–1086). IFN also inhibits cell fusion caused by viruses, such as Sendai virus (Tomita et al., 1981, J. Gen. Virol. 55:289–295).
To counteract the deleterious effects of IFN-induced cellular defense mechanisms, many eukaryotic viruses have evolved mechanisms to block the activity of IFN-induced proteins. A number of viruses produce RNAs which override the IFN-induced host cell shut off of translation, for example, the adenovirus produced VAI RNA molecule, human immunodeficiency virus (HIV) TAR region, and the Epstein-Barr virus produced EBER-1 RNA molecule (Katz et al., 1987, Embo. J. 6:689–697; McMillan et al., 1995, Virology 213:413–424; Carroll et al., 1993; Beattie et al., 1995, Virology 210:254–263; Beattie et al., 1991, Virology 183:419–422). A number of viruses such as the vaccinia virus K3L protein, which binds to and inhibits IFN-induced PKR protein kinase (Gale Jr. et al., 1996, Mol. Cell. Biol. 16:4172–4181). Influenza virus, recruit cellular factors, such as P58 which binds to and blocks PKR protein kinase (Lee et al., 1990, Proc. Natl. Acad. Sci. 87:6208–6212; Lee et al., 1994, Mol. Cell. Biol. 14:2331–2342).